1. Field of the Invention
The present invention relates to a method for selecting a light source, such as semiconductor lasers and integrated light source for optical modulator, used in optical communication systems, and relates in particular to a selection method to determine dispersion tolerance quality of the light source.
2. Description of Related Art
One of indexes showing performance of semiconductor lasers used as a light source in optical communication systems is transmission capability, i.e., dispersion tolerance, and a light source having a superior dispersion tolerance is selected and used to operate a communication system. A conventional method for selecting a semiconductor laser having a superior dispersion tolerance utilizes a device shown in FIG. 10 to measure the post-transmission power penalty of an optical fiber path to determine its quality.
FIG. 10 is a schematic diagram of a conventional evaluation system used to select the dispersion tolerance quality. As shown in FIG. 10, a dispersion tolerance evaluation device is composed of: an NRZ (non-return-to-zero) signal generator 51 for supplying NRZ signals to a semiconductor laser (referred to as the element hereinafter) 53 to be evaluated through an electric amplifier 52; optical fiber 45; EDFAs (Erbium doped fiber amplifier) 44; a wavelength filter 43; a receiver disk 42; an error rate detector 57; a sampling oscilloscope 54 for post-transmission waveform observation; and a computer 56 for controlling the error rate detector.
NRZ-modulated light output from the element 53 propagates through the optical fiber 45 while receiving loss compensation by the EDFA 44, and after ASE noise is eliminated by the wavelength filter 43, arrives in the receiver disk 42. The error rate of signals detected by the receiver disk 42 is evaluated in the error rate detector 57, and a bit error rate curve is measured in real-time. Further, the bit-error rate of the optical signal just after emission from the element 53, that is, the bit-error rate of the optical signal before it propagates through the optical fiber 45 is separately measured in real-time. From the measured data of bit error rates before or after transmission, the power penalty is determined, and an element that produces results lower than a predetermined power penalty value is selected as an acceptable product. In FIG. 10, the arrangement shown for dispersion tolerance evaluation is for a 600 km transmission path, but in practice, the fiber length is varied according to the dispersion tolerance quality of the element.
However, according to the conventional method for evaluating the dispersion tolerance, actual transmission experiments must be carried out, thus it is necessary to provide incidental facilities such as optical fibers, EDFAs, wavelength filter, receiving disk (RX) and the like. Also, depending on the dispersion tolerance of an element to be required, the fiber length must be varied for each test. Furthermore, to measure the bit error rate (BER), it is necessary to devote about 15 minutes for each element. Therefore, the conventional method for evaluating the dispersion tolerance presents problems of excessive facility cost and lengthy selection process.
It is an object of the present invention to eliminate the need for facilities such as EDFAs, optical fibers, wavelength filter, receiving disk, error rate detector, and the like for measuring the dispersion tolerance of a light source, and to significantly shorten the selection time required in evaluating the light source.
A first aspect of the present invention provides a method for selecting a light source for optical communication system comprising the steps of: measuring time division chirping characteristics and optical response waveforms of the light source responding to a fixed strength random pulse signal; performing a simulation of a transmission process based on measured data; computing a selection parameter as an index for determining a dispersion tolerance quality of the light source according to a computed post-transmission waveform of an optical signal that propagated through an optical fiber path; and deciding the dispersion tolerance quality of the light source based on values of the selection parameter.
A second aspect of the present invention provides a device for selecting a light source for optical communication system comprising: a measuring section for measuring time division chirping characteristics and optical response waveforms of the light source responding to a fixed strength random pulse signal; and a simulation section for computing a post-transmission waveform of an optical signal according to measured data, and computing a selection parameter as an index for determining a dispersion tolerance quality of the light source; and determining the dispersion tolerance quality of the light source by comparing the selection parameter with a predetermined selection criterion.
In the above aspects, the selection parameter is a value of an eye opening penalty Peye computed according to an equation:
Peye=10xc2x7log (Q/QB.B) 
(Notice: Q refers to a Q-factor computed from a post-transmission waveform of an optical signal resulting from a transmission simulation process, and QB.B refers to a Q-factor computed from a pre-transmission waveform of the optical signal.) or a Q-factor computed from a post-transmission waveform resulting by a transmission simulation process.
The present invention not only reduces the number of selection steps but is able to simulate the transmission process through the optical fiber itself so that it offers not only a freedom to choose transmission distance and dispersion characteristics through the fiber path but also an advantage that lesser incidental facilities such as optical fibers and EDFAs are needed for the selection process.
According to the above aspects, the present invention enables to replace actual experimentation of signal transmission through an optical fiber path with a simulation process, so that the present invention not only enables to freely select the transmission distance and dispersion characteristics of the fiber path, but also eliminates the necessity for items of experimental facility, such as EDFAs, optical fibers, wavelength filter, receiving device, error detector and the like. Also, a selection parameter for indexing the dispersion tolerance can be computed readily by simply changing the values of transmission distance (fiber length L) and the secondary group velocity dispersion xcex2, according to the dispersion tolerance required, so that a dispersion tolerance quality required for an application can be easily and speedily determined. Furthermore, because a simulation process itself is completed in short time, the selection time can be significantly reduced compared with an actual experimental evaluation process.